THREE/4 Asteroids

STEP ONE: Determine predominant asteroid type (in the belt) on 3.4.1

STEP TWO: Determine total asteroid mass of the belt on 3.4.2

Table 3.4.1 Belt Type

1d10+2   Type
≤0     M (metallic)
1-7     S (silicate)
8-12     C (carbonaceous)
≥13     I (icy)

+6 if in outer zone
-2 if closer than life zone
-1 if belt density 0.61 to 0.80
-2 if belt density 0.81 to 1.00
-3 if belt density 1.01 to 1.20
-5 if belt density above 1.20

Table 3.4.2 Asteroid Mass

1d10   Type
≤4     1d10 x 0.0001
5-6     1d10 x 0.001
7-8     1d10 x 0.01
9-10     1d10 x 0.1
≥11     1d10 x 1

+ System Abundance
+2 if outer zone
-1 if inner zone
-1 if system age older than 7 Gy
+2 if belt around 2 or more stars


Here, we define four basic types of asteroids.

M: Metallic. These asteroids are made up largely by metals and are differentiated. They can be thought of as the remnants of metal cores of destroyed proto-planets.

S: Silicate. These asteroids can be differentiated (part of old crust/mantles) but most are primitive.

C: Carbonaceous. These asteroids are dark and include carbon compounds and water bound to silicates. They are primitive.

I: These asteroids are mostly made up of ices and various frozen gasses. When heated they become comets. These asteroids are only found in the outer zone on a permanent basis.

Any asteroid belt in the life zone or closer than life zone orbit will have both M & S type asteroids, and perhaps a few C-types. Any asteroid belt in the inner zone but outside the life zone will have all three sort. In the outer zone, C & I-types will be the ones found.


There are many sub-types of asteroids. Some may be intact smaller but differentiated bodies, with a core like any small planetoid. These are rare, and the older a system is the greater is the chance that they have been hit by other asteroids and broken up.


Asteroid belts are generally as wide as half the distance between the next inner and outer orbit, but if any of the orbits contain a big gas giant or superjovian it may be significantly thinner. Most asteroid belts contain rather small amounts of material compared to planets, but large outer belts may have a lot of material. This generation do not cover Oort clouds (which are much larger and also can have much more mass), nor do it cover the protoplanetary disks and planetoids of very young systems. These cases allow for much larger belts. Belts around binaries are also often larger.


Most asteroids are small - less than 1km across - but there will almost always be larger objects, up to several hundred of kilometers across or in exceptional cases, perhaps even above 1000 kilometers in radius. The denser the belt is the less likely it is large objects will survive, of course, and time tend to favor creation of small objects.


These are not asteroid belts but usually features of young (<1Gy) systems. A dust disk typically lies tens of AUs from the star and has about 0.1 to 0.001 Earth masses total.


Only a few large asteroids are spherical - most are irregular bodies. Some may even have small moons or act as double chunks. They have numerous small craters from smaller impacts. Some asteroids are grayish, while C-types tend to be dark. Icy bodies are often reddish.


While most asteroids are within the main belts and in the normal orbital plane, there are always those much more eccentric and/or inclined. Asteroid belts do not form a "sphere" around a star, however. Too few have inclined orbits for that effect. (Oort clouds, on the other hand, may be more spherical)


A differentiated body is a body which has been extensively heated and thus have different composition in core than crust. Asteroids of differentiated nature may be remnants of destroyed larger bodies or small mini-planets in their own right. Primitive asteroids have condensed directly from the young system, and while they have been heated somewhat leading to a certain geological evolution they are not really differentiated.